1. Field of Invention
This invention relates to processes and apparatus for forming cyanoacetate and cyanoacrylate.
2. Description of Related Art
Monomer and polymer adhesives are used in both industrial (including household) and medical applications. Included among these adhesives are the 1,1-disubstituted ethylene monomers and polymers, such as the xcex1-cyanoacrylates. Since the discovery of the adhesive properties of these monomers and polymers, they have found wide use due to the speed with which they cure, the strength of the resulting bond formed, and their relative ease of use. These characteristics have made the xcex1-cyanoacrylate adhesives the primary choice for numerous applications such as bonding plastics, rubbers, glass, metals, wood, and, more recently, biological tissues.
It is known that monomeric forms of xcex1-cyanoacrylates are extremely reactive, polymerizing rapidly in the presence of even minute amounts of an initiator, including moisture present in the air or on moist surfaces such as animal (including human) tissue. Monomers of xcex1-cyanoacrylates are anionically polymerizable or free radical polymerizable, or polymerizable by zwitterions or ion pairs to form polymers. Once polymerization has been initiated, the cure rate can be very rapid.
Cyanoacrylate monomers are generally produced by forming polycyanoacrylate and then cracking the polycyanoacrylate polymer to produce monomeric cyanoacrylate. Polycyanoacrylate is commonly produced by first reacting cyanoacetate with formaldehyde, or a functional equivalent such as the polymeric form paraformaldehyde, in the presence of a base. The base acts as a catalyst for the reaction between the cyanoacetate and paraformaldehyde. This reaction in the presence of the catalyst produces formaldehyde and polycyanoacrylate.
An exemplary known process for forming cyanoacetate is described in Japanese Patent Appln. Laid Open No. 60-95760. The disclosed process synthesizes cyanoacetate in a wiped-film evaporator using dibutyltin oxide. Titanium tetraisopropoxide can be used as a catalyst. A continuous process is disclosed.
A method for preparing cyanoacetic acid higher ester is described in U.S. Pat. No. 5,698,730 to Nakamura et al. This method includes subjecting a cyanoacetic acid ester to a transesterification reaction in the presence of a tin compound.
These known methods for producing cyanoacetate do not convert a lower homologue cyanoacetate to a higher homologue cyanoacetate in a continuous process.
Known processes for cracking polycyanoacrylate to produce cyanoacrylate monomer have many disadvantages. One disadvantage of the known processes is that they produce unwanted byproducts that must be separated from the monomer product. Such byproducts include, for example, alcohols and cyanoacetate. Often it is difficult and/or costly to remove such byproducts from the monomer product, resulting in impure product, increased production cost and time, and/or reduced yield. In addition, such impure product may not be suitable for certain uses, such as, for example, animal and human use.
Another disadvantage of known processes for cracking polycyanoacrylate is that during the cracking process, the temperature of the polycyanoacrylate is not sufficiently controlled to avoid the formation of what is referred to as xe2x80x9chot monomer.xe2x80x9d If the polymerization temperature is too high, monomer tends to polymerize on surfaces of the reactor, causing a buildup in the reactor. In addition, if the polycyanoacrylate polymer is cracked at too high of a temperature, the resulting monomer may not be stable. This problem again can result in an impure product, increased production cost and time, and/or decreased yield.
Further, in known processes of cracking polycyanoacrylate, the entire polymer is exposed to a high temperature for a long period of time, typically for as long as about eight hours or more. Consequently, side products are also formed during cracking. Hot monomers can form, which are overly reactive and have a tendency to polymerize. Consequently, such hot monomers have an insufficiently short shelf life.
U.S. Pat. No. 3,728,373 to Imohel et al. discloses a method for producing cyanoacrylic acid esters by depolymerizing polycyanoacrylic acid esters in a continuous process. In this process, polymer is admixed with an inert liquid having a high boiling point and a polymerization inhibitor. Depolymerization is conducted in a reaction zone at a temperature of 150-250xc2x0 C. and a vacuum pressure of 0.5-40 mm Hg. Wiper blades in the reaction zone distribute the mixture in the form of a thin film having a thickness of up to 5 cm. Cyanoacrylic acid esters that are released from the reaction zone are passed to a condenser and collected in receivers. A dispersion of polycyanoacrylic acid esters and other components is collected as a thin layer on the surface of Woods metal filled in a vessel. The temperature of the metal surface is 180-240xc2x0 C. in the vessel. Vapors of the monomeric cyanoacrylic acid ester are condensed in a cooled receiver.
U.S. Pat. No. 4,986,884 to Arlt et al. discloses a process for the production of monomeric xcex1-cyanoacrylates. The Arlt process produces monomeric cyanoacrylates by pyrolyzing poly-xcex1-cyanoacrylates. The monomer is subsequently distilled in a distillation column. The distillation is carried out in a counter current apparatus at reduced pressure over a plurality of separation stages. Polymerization inhibitors are fed continuously to the counter current at an uppermost separation stage. Liquid monomer is fed in at places where the condensation can collect in the apparatus.
U.S. Pat. No. 5,436,363 to Wang et al. discloses a method for making cyanoacrylate by the depolymerization of poly(alkyl-xcex1-cyanoacrylate). In the Wang process, a reaction mixture containing poly(alkyl-xcex1-cyanoacrylate), a polymerization inhibitor and a solvent is fed into a film evaporator to depolymerize the feed material. The film evaporator is operated at a pressure of 10 mm Hg vacuum and a temperature of about 200xc2x0-260xc2x0 C. A first gas stream and a first residual liquid stream are produced from the film evaporator. The first residual liquid stream is passed to a collector. The first gas stream is fed to an intermediate heat exchanger having a very high temperature of about 150xc2x0 C. to produce a second gas stream and a second residual stream of high-boiling residue. The second gas stream is fed from a liquid collector to a second heat exchanger (condenser) to form alkyl-xcex1-cyanoacrylate monomer.
Wang utilizes classical wiped-film evaporator technology. However, such wiped-film devices cannot operate at low pressure levels, such as at micron pressure levels, i.e., pressures as low as about 10xe2x88x923 mbar. Consequently, wiped-film evaporators heat the polymer to a relatively high temperature. As explained above, however, heating the polymer to high temperatures is disadvantageous, as it produces hot monomers and unwanted byproducts.
Thus, there is a need for a process that can produce cyanoacrylate monomer from polycyanoacrylate without incurring the above-described disadvantages of known processes. There is also a need for a continuous process for preparing cyanoacetate that converts a lower homologue cyanoacetate to a higher homologue cyanoacetate.
This invention provides processes and apparatus for the production of cyanoacrylate from polycyanoacrylate that satisfy one or more of the above-described needs.
Processes and apparatus for producing cyanoacrylate according to this invention can produce cyanoacrylate in a continuous manner. The cyanoacrylates that can be produced include alkyl cyanoacrylates, as well as other types of cyanoacrylates.
In addition, processes and apparatus for producing cyanoacrylate according to this invention can reduce exposure of polycyanoacrylate to high temperatures. Consequently, the above-described problems associated with exposing polycyanoacrylate to high temperatures, that occur in known processes and apparatus for forming cyanoacrylate, can be avoided.
Embodiments of this invention can produce cyanoacrylate with reduced waste, increased efficiency and at reduced cost.
This invention also provides processes and apparatus for the continuous production of cyanoacetate. Embodiments of the processes can convert a lower homologue cyanoacetate to a higher homologue cyanoacetate. The cyanoacetate produced by these processes can subsequently be used in embodiments of the above-described processes for continuously producing cyanoacrylate according to this invention.